The present invention relates to a heat-sealable connector sheet or, more particularly, to a connector sheet for making electrical connection between the electrode terminals on an electronic device, such as liquid crystal display units, electroluminescence display units, light-emitting diodes, electrochromic display units, plasma display units and the like, and the electrode terminals of the driving circuit therefor formed on a circuit board or between two sets of electrode terminals on different electric circuit boards.
It is well established that electrical connection between two sets of electrode terminals as mentioned above can be achieved by using a heat-sealable connector sheet consisting of an electrically insulating substrate sheet having flexibility and a patterned layer formed by printing thereon with an electroconductive paste which is a composite of an insulating adhesive resin blended with fine particles having electric conductivity in such an amount that the electroconductive layer formed therefrom has anisotropic electroconductivity only in the direction perpendicular to the plane of the layer (see, for example, Japanese Patent Publications 55-38073 and 58-56996).
The heat-sealable connector sheets of this type, however, cannot fully comply with the demand in the modern electronic technology which is constantly under a trend toward more and more compact design of the electronic instruments in which the pitch of the line-wise patterned electrode terminals in an array is decreasing to 0.3 mm, to 0.2 mm or even finer. When electrical connection is made between such finely patterned electrode terminals by using a heat-sealable connector sheet of the above described type, namely, short-circuiting is sometimes unavoidable between adjacent two terminals as a consequence of displacement of the electroconductive particles out of the proper position. This problem has been at least partly solved by the teaching in Japanese Patent Kohyo 62-500828 and Japanese Patent Kokai 62-154746 according to which an overcoating layer is provided over the whole surface of the connector sheet with a melt-flowable insulating adhesive. When electrode terminals are connected by using such a connector sheet under heating and pressure, melt of the melt-flowable adhesive is driven off from the surface of the conductive line patterns to form a pool of the melt between the conductive lines to ensure good insulation between the conductive lines.
The above proposed improvement in a conventional heat-sealable connector sheet, however, is far from a complete solution of the problems. To explain it, the electroconductive particles dispersed in the insulating adhesive matrix to form a conductive paste are usually formed from a metal or a carbonaceous material having high rigidity so that the particles cannot comply with the deformation or displacement of the insulating flexible substrate, electroconductive layer and the insulating overcoating adhesive layer in conducting heat-sealing with heating under pressure. The particles also may be subject to a microscopic displacement due to the residual stress in the layers after heat sealing. Therefore, troubles are sometimes caused in the assembly of electrode terminals constructed by using such a heat-sealable connector sheet such as failure of electrical connection, increase in the electric resistance between the thus connected terminals and the like during use resulting in a loss of reliability of the electric connection.
The above described problem could of course be solved by replacing the electroconductive particles of high rigidity with particles of a polymeric material having flexibility. Indeed, a proposal is made for the use of particles of an elastic polymeric material having a plating layer of a noble metal on the surface thereof to impart electroconductivity. These noble metal-plated elastomer particles, however, have another problem that microscopic cracks are sometimes formed in the plating layer as a consequence of the difference in the hardness and other physical properties between the core particles and the surface-plating layer so that a trouble of electric corrosion may take place due to a trace amount of residual electrolyte on the thus exposed surface of the core particles remaining after the plating treatment of the particles with the noble metal. Needless to say, the expensiveness of such noble metal-plated particles is another disadvantage to prohibit industrialization of this technology.